1. The Technical Field
The invention relates generally to electrical machines, such as motors and generators, and, more particularly, to electrical machines having hybrid rotors, i.e, both electric and permanent magnet excitation.
2. The Prior Art
Lundell-type electrical machines are well-known in the art. For example, Lundell-type electrical generators have long been used in the automotive industry to provide electrical power for automobiles and trucks. Due to consumer demand for power consuming convenience and luxury items, such as high-powered sound systems, power windows, and the like, as well as the need for the complicated control systems required to meet government emission and safety standards, electrical demands on modem automobiles have increased substantially over the years. Although the output of a conventional Lundell-type generator can be increased to meet the increased electrical demand, the additional electrical output comes at the expense of additional size and weight. Since underhood space on modern automobiles is limited, the use of a physically larger generator to meet a vehicle""s increased electrical demands might not be an acceptable design solution in some cases. Further, as vehicle weight increases, fuel economy and performance decrease.
An electrical generator""s output can be increased without a proportional increase in size and weight by using a permanent magnetic field to supplement the conventional electrically-generated rotor field. U.S. Pat. Nos. 4,959,577 and 5,483,116 disclose hybrid Lundell-type generators having a plurality of discrete permanent magnet segments located between the interleaved fingers of the rotor claw segments. However, the designs disclosed by the foregoing references involve complicated arrangements of magnets and magnet holders, thus making the generator difficult to assemble.
It would be desirable to provide an electrical generator which provides high specific output, using proven design principles, in a relatively lightweight, compact, easy-to-assemble package.
The invention is a hybrid Lundell-type electrical machine characterized by a rotor whose magnetic field is established using one or more conventional field coils and one or more permanent magnets.
A conventional Lundell-type generator rotor includes a bobbin-wound field coil mounted on a rotor shaft to generate a magnetic field. The magnetic field flux is transferred through two claw-type rotor segments forming the north and south poles, respectively, of the magnet thus-created. The magnetized rotor assembly spins inside a stator having a number of windings which are xe2x80x9ccutxe2x80x9d by the magnetized rotor""s flux lines so as to induce an electrical current in the stator windings.
The output of a Lundell-type generator is a function of the rotor magnetization, among other parameters. Rotor magnetization is a function of the magnetic field strength in the rotor, which, in turn, is a function of the field coil excitation current. That is, by increasing the field coil excitation current, and thereby increasing the induced magnetic field strength in the claw segments, the rotor magnetization can be increased. However, the magnetization of the claw segments can be increased only up to a certain level, based on the size and material composition of the claw segments, beyond which the claw segments become magnetically saturated. Once the claw segments become saturated, their magnetization does not continue to increase with increased magnetic field strength. Therefore, as a practical matter, the maximum rotor magnetization in a conventional Lundell-type generator is a function of the rotor""s size and, more particularly, the claw segments"" size.
Permanent magnets can be used in lieu of a field coil to magnetize a generator rotor, and they can provide greater rotor magnetization than a field coil of comparable size. However, the output voltage of a generator using only a permanent magnet to establish rotor magnetization is not easily controlled.
The present invention is directed to a hybrid electrical machine whose rotor magnetization is established using a combination of one or more electrically excitable field coils and one or more permanent magnets; the remainder of the generator may be conventional. The permanent magnet provides a base level of rotor magnetization. The rotor""s magnetization can be increased above the base level of magnetization provided by the permanent magnet, subject to the limitations discussed above, by electrically exciting the field coil. Since a permanent magnet of a given size can effect a greater level of rotor magnetization than a field coil/claw segment assembly of the same size, a hybrid rotor according to the present invention can achieve a higher level of magnetization than a conventional rotor of the same size. Alternatively, a hybrid rotor according to the present invention having a predetermined level of magnetization can be smaller and lighter than a conventional Lundell-type generator rotor having the same level of magnetization.
A first embodiment of a hybrid rotor according to the present invention comprises an otherwise conventional Lundell-type generator rotor (i.e., a rotor shaft having a field coil and two claw segments, each claw segment having a plurality of axially-extending fingers) having one or more radially-magnetized permanent magnets located about the periphery of each of the claw segments. In a preferred embodiment, a ring magnet is provided for each claw segment (i.e. each hybrid rotor includes two ring magnets). In an alternate first embodiment, a plurality of discrete magnets may be used in place of one or both of such ring magnets. In any case, the magnet or magnets associated with each claw segment have a number of magnetic poles equal to twice the number of fingers associated with the respective claw segment.
In a second embodiment of the invention, the hybrid rotor includes first and second claw segments mounted on a shaft, as would a conventional Lundell-type generator rotor. The rotor further includes a third claw segment located between the first and second claw segments. The third claw segment has fingers extending axially in both directions, towards both the first and second claw segments, such that the fingers of the third claw segment are interleaved with the fingers of the first and second claw segments. A first field coil is located between the first and third claw segments and a second field coil is located between the second and third claw segments. One or more permanents magnets are located about the periphery of the first and second claw segments, in the same manner as discussed above for the first embodiment. In an alternate second embodiment, one or more permanent magnets are located about the periphery of the third claw segment, but not the first and second claw segments. In another alternate second embodiment, one or more permanent magnets are located about the periphery of the first, second, and third claw segments.
In a third embodiment of the invention, the hybrid rotor includes first and second claw segments mounted on a non-magnetic shaft. The rotor further includes a third claw segment located between the first and second claw segments. The third claw segment has fingers extending axially in both directions, towards both the first and second claw segments, such that the fingers of the third claw segment are interleaved with the fingers of the first and second claw segments. A first field coil is located between the first and third claw segments, and an axially-magnetized permanent magnet comprising a ring magnet or a plurality of discrete magnets is located between the second and third claw segments. In an alternate third embodiment, one or more radially-magnetized permanent magnets also may be located about the periphery of the first, second, and/or third claw segments.
This application is being filed contemporaneously with related U.S. patent application Ser. No. 09/650,334 entitled xe2x80x9cHybrid Electrical Machine with Axial Flux Magnet,xe2x80x9d and related U.S. patent application Ser. No. 09/649,306 entitled xe2x80x9cHybrid Twin Coil Electrical Machine,xe2x80x9d both of which are owned by common assignee Delphi Technologies, Inc.